1. Field of the Invention
The present invention relates to an optical pickup device. More specifically, the present invention relates to an optical pickup device for performing data recording/reproduction on an optical disk with a cover layer formed on a signal recording layer.
2. Description of the Prior Art
In recent years, an optical disk has been widely used as a medium for recording video data, audio data, computer data, etc. The need for an optical disk with high recording density and high capacity has been more and more increasing.
An optical disk has a cover layer through which light enters a signal recording layer. Recording/reproduction is carried out by irradiating the light to the signal recording layer through the cover layer. An object lens is designed in such a manner that, when a film thickness of the cover layer meets a reference value (standard value in the specification), a spherical aberration is a minimum on the signal recording layer. Thus, a spherical aberration occurs if the thickness of the cover layer differs from the reference value, for example, if a plurality of signal recording layers exist on one side of the optical disk, or if the cover layer exhibits manufacturing variations in film thickness.
According to the conventional optical pickup device disclosed in Japanese Patent Laying-open No. 2003-77142 (G11B 7/085), a beam expander composed of two convex lenses or one convex lens and one concave lens is located between a collimator lens and an object lens in order to correct such a spherical aberration that occurs due to a deviation of the cover layer in film thickness from the reference value. The beam expander changes a distance between the two lenses by driving one of the lenses with a stepping motor so as to adjust the emitted light in such a manner that it becomes converged light or diffused light. If the film thickness of the cover layer of the optical disk is small, for example, the beam expander lets converged light into the object lens. As a result, a spherical aberration due to the thin cover layer is cancelled by a spherical aberration that has occurred on the object lens, which causes almost no spherical aberration on the signal recording layer.
However, if the stepping motor for driving the lens in the beam expander is to be provided within an optical pickup device, there is a problem where the optical pickup device capable of correcting a spherical aberration becomes large in size.